1. Field of the Invention
The present invention relates to a nickel hydride secondary battery.
2. Description of the Related Art
From the viewpoint that a nickel hydride secondary battery has high capacity and excels in environmental safety compared to a dry-cell battery or a nickel-cadmium secondary battery, the nickel hydride secondary battery has been used for a variety of applications such as various portables and a hybrid electric vehicle. The capacity means quantity of electricity that can be derived from the battery.
A hydrogen absorbing alloy used for a negative electrode of the nickel hydride secondary battery has a feature capable of absorbing a large amount of hydrogen gas, and contributes to increase in capacity of the nickel hydride secondary battery.
The negative electrode embedded in the nickel hydride secondary battery is generally manufactured by a slurry method. According to the slurry method, firstly, hydrogen absorbing alloy powder and conductive material powder are dispersed in water as a solvent, and further, thickening agent powder is injected therein, and these are kneaded to prepare a negative-electrode mixture slurry. The obtained negative-electrode mixture slurry is applied to a negative-electrode substrate of a metal porous body or the like. After that, the negative-electrode substrate supporting the negative-electrode mixture slurry undergoes a drying step, a rolling step, and a cutting step to be formed into the negative electrode (for example, refer to Unexamined Japanese Patent Publication No. 10-012231). In the slurry method, a thickening agent is used to adjust the negative-electrode mixture slurry to have an appropriate viscosity such that the negative-electrode mixture slurry is not dripped from the negative-electrode substrate in manufacturing processes.
While having high capacity, a conventional nickel hydride secondary battery has large self-discharge. Thus, remaining capacity decreased after having been left for a long time, and the battery needed to be charged just before use.
Therefore, in order to improve self-discharge characteristics, various types of research have been conducted on the nickel hydride secondary battery. As a result, a negative-electrode additive agent for suppressing self-discharge was added to a negative electrode, and a self-discharge suppression-type nickel hydride secondary battery was developed. It is to be noted that, when a degree of remaining capacity after a battery has been left for a long time is defined as self-discharge characteristics, the expression of self-discharge characteristics are higher or better means that remaining capacity after having been left for a long time is closer to capacity in the case of full charge.
The self-discharge suppression-type battery as described above has the advantage in that, if a user charges the battery in advance, occurrence frequency of situations requiring recharge just before use can be reduced because the reduced amount of the remaining capacity after having been left for a long time is small. By utilizing the advantage, the self-discharge suppression-type nickel hydride secondary battery is an excellent battery which has both of ease of use as if it were a dry-cell battery and capacity equal to or higher than that of a dry-cell battery.
The above-described negative-electrode additive agent is used by being injected in the negative-electrode mixture slurry. However, while having a property of suppressing self-discharge of the battery, the negative-electrode additive agent has also a property of inhibiting the function of the thickening agent in the negative-electrode mixture slurry. Thus, the negative-electrode mixture slurry to which the above-described negative-electrode additive agent is added does not have a sufficient thickening property by the thickening agent, and the viscosity of the negative-electrode mixture slurry significantly decreases. As a result, when applying the negative-electrode mixture slurry to the negative-electrode substrate, the negative-electrode mixture slurry is dripped and it becomes difficult to manufacture the negative electrode.
Therefore, the amount of the thickening agent is generally increased so as to increase the viscosity of the negative-electrode mixture slurry such that the negative-electrode mixture slurry can be successfully supported on the negative-electrode substrate. When the amount of the thickening agent increases in this manner, the viscosity of the negative-electrode mixture slurry increases, and the dripping of the slurry is suppressed, and thus, the negative electrode can be manufactured as is conventionally done. That is, when the negative-electrode additive agent for suppressing self-discharge is added to the negative-electrode mixture slurry, the amount of the thickening agent needs to be increased compared to ever before.
However, since the thickening agent does not directly contribute to the battery reaction, when the amount of such a thickening agent increases, the amount of the hydrogen absorbing alloy contributing to the battery reaction relatively decreases, and a value of maximum capacity of the battery is lowered. That is, the nickel hydride secondary battery to which the negative-electrode additive agent for suppressing self-discharge is added can improve self-discharge characteristics, but capacity of the battery has no choice but to be sacrificed to some extent with increase in the thickening agent.
As just described, although the conventional self-discharge suppression-type nickel hydride secondary battery has capacity equal to or higher than that of a dry-cell battery, the value remains insufficient. That is, in the nickel hydride secondary battery, both of improvement in self-discharge characteristics and increase in capacity of the battery have not been sufficiently achieved yet.